1. Field of the Invention
The present invention relates to a divisional exposure apparatus. More particularly, the present invention relates to a divisional exposure apparatus which divides the exposure region of a photosensitive element and exposes each divided exposure region using one of a plurality of exposure devices.
2. Related Art
A conventional exposure apparatus which exposes a photosensitive element such as a drum-shaped photosensitive element by exposure beams commonly uses a semiconductor laser beam as a light source. In such a conventional exposure apparatus, which exposes a large exposure region of a photosensitive element, a plurality of small, inexpensive exposure devices are used in combination in order to divide a large exposure region and expose each divided exposure region using one of the small exposure devices. Exposure of a large exposure region by a single exposure device will require large, expensive optical parts including condenser lenses, making the exposure apparatus remarkably expensive.
As shown in FIG. 4, such a conventional divisional exposure apparatus includes two exposure devices 50 and 52 having a parallel construction: the exposure devices 50 and 52 comprises the same elements, namely, laser beam generators 54 and 56 for generating a laser beam based on an externally provided signal, rotating polygon mirrors 58 and 60 for polarizing the laser beam generated by the laser beam generators 54 and 56, reference signal generation circuits 62 and 64 using crystal oscillation circuits, drive circuits 66 and 68 for rotatably driving the rotating polygon mirrors 58 and 60 at a steady speed based on the signals sent from the reference signal generation circuits 62 and 64, and detectors 70 and 72 for detecting laser beams prior to laser beam exposure based on externally provided timing information.
In operation, the drive circuits 66 and 68 rotate the rotating polygon mirrors 58 and 60 at a steady speed based on the signals sent from the reference signal generation circuits 62 and 64 and the detectors 70 and 72. Then, the laser beam generated by the laser beam generators 54 and 56 are detected by the detectors 70 and 72. The detection signals sent from detectors 70 and 72 are used as trigger signals of not-shown exposure information devices. The laser beam generators 54 and 56 generate laser beam based on the information sent from the exposure information devices. Subsequently, the rotating polygon mirrors 58 and 60 polarize the laser beams to scan the photosensitive element.
However, in the above conventional exposure apparatus, the scanning lines of the exposure device 50 and 52 sometimes do not connect smoothly, causing displacement therebetween as shown in FIGS. 5A-B. This is because the rotating polygon mirrors 58 and 60 are simply driven to rotate at a steady speed by the respective drive circuits 66 and 68. While the laser beams scan the photosensitive element in a direction parallel to the axis of the drum-shaped photosensitive element (referred to as the primary scanning direction hereinafter), the photosensitive element travels in a direction perpendicular to the scanning direction (referred to as the secondary scanning direction hereinafter), exposing the scanning region on the photosensitive element. Therefore, occasionally the scanning line of the exposure device 50 does not connect smoothly with that of the exposure device 52, causing displacement therebetween. For instance, as shown in FIG. 5A, if the scanning directions of the exposure devices 50 and 52 are slanting in accordance with the traveling speed of the photosensitive element, scanning lines a1 and a2 perpendicularly cross the traveling direction of the photosensitive element at a right angle. In this case, if the exposure timing of the exposure devices 50 and 52 does not match for some reason, for example, if the rotation speed of the rotating polygon mirror 60 of the exposure device 52 is slowed down by the malfunction of the drive circuit 68, the scanning lines a1 and a2 of the exposure devices 50 and 52 do not connect smoothly, distorting the image.
If, on the other hand, the primary scanning directions of exposure devices 50 and 52 are arranged perpendicularly to cross the traveling direction (secondary scanning direction) of the photosensitive element, the scanning lines b1 and b2 are made slanting at the secondary scanning direction. In the event that the exposure timing of the exposure devices 50 and 52 does not match for some reason, for example, if the rotation speed of the rotating polygon mirror 60 of the exposure device 52 is slowed down by the malfunction of the drive circuit 68, the scanning lines a1 and a2 of the exposure devices 50 and 52 do not connect smoothly, distorting the image.
Another problem of the conventional divisional exposure apparatus is that the degree of exposure sometimes changes too much where the exposure beams connect. More particularly, if the two exposure beams neatly abut on each other without displacement in the middle of a large exposure region, the large exposure region is uniformly exposed as shown in FIG. 14A. As shown in FIG. 14B, however, if the two exposure beams overlap each other, the overlapping region T3 is exposed at a degree twice as high as the other regions. On the other hand, as shown in FIG. 14C, if there is a gap between the two beams on the photosensitive element, the gap becomes a non-exposure region T4 in the middle of the large exposed region T0.
The overlapping exposure (double exposure) or the non-exposure may cause misinterpretation of the information and hinder printing of a high-quality image. Despite attempts made to adjust the seam of the two beams, it has been difficult to completely overcome the problem, thus preventing double exposure caused by overlapping or non-exposure.